Automated multiple sample processing for well type radioactivity counters

ABSTRACT

A means for radioactivity analysis of samples containing a substance present at least partially in a radioactive form, but present in varying quantities in different samples. The radioactivity in the samples is automatically compared with the radioactivity level of a reference set of samples containing the aforesaid substance in a maximum concentration of radioactive to non-radioactive forms. Concurrent analysis is performed on multiple samples in a sample set to reduce the sample processing time.

United States Patent Obrycki an. 12, 1974 [54] AUTOMATED MULTIPLE SAMPLE 3,609,361 9/1971 Shevick 250/ 106 SC g gg F WELL TYPE FOREIGN PATENTS OR APPLICATIONS CA I I Y COUNTERS 1,111,121 4/1968 Great Britain 250/106 SC [75] Inventor: Raymond F. Obrycki, Mt. Prospect,

Primary Examiner-Harold A. Dixon 1 Attorney, Agent, or Firm-Walter C. Ramm; Peter J. 73 A .G.D.S I&C.Ch ,Ill. sslgnee ear e o lcago Sgarbossa; Charles H. Thomas, Jr. [22] Filed: Mar. 24, 1972 1 [21] Appl. No.: 237,662 [57] ABSTRACT A A means for radioactivity analysis of samples contain- [52] U 8 Cl 250/362 250/363 250,366 ing a substance present at least partially in a radioac- [5.1.], iii-1C. 6 j 39/18 tive form, but present in varying quantities in different [58] Field SC 71 R samples. The radioactivity in the samples is automatically compared with the radioactivity level of a refer- 56] References Cited ence set of samples containing the aforesaid substance in a maximum concentration of radioactive to non- UNITED STATES PATENTS radioactive forms. Concurrent analysis is performed gamer on multiple samples in a sample set to reduce the samutmann 3,395,280 7/1968 Dietrich 250/106 SC ple processing tune. 3,381,130 4/1968 Nather 250/106 SC 13 C, 3 Drawing Figures RESID SUBTR PULSE PULSE COUNTER IIIII" 30 READOUT 34 9 8b l RlZSlD MEMORY 8c ISIGNAIL PULSEk COUNTER 007MB ACC RACY DI I/IDER Io SAMPLE J CONTROLLER REFERENCE CONTROLLER l3 CLOCK Q CLOCK PATENYEDHARQW 3,796,879

SHEEI 3. 0F 2 PM PM PM 3 3 3 RESID SUBTR PULSE PULSE COUNTER HEIGHT 4 ANAL RESID SUBTR 3O READOUT 34 9 x Sb L 30 RESID r; MEMORY 8C MASTER couNTER 22 I I PULSE y SIGNAL COUNTER 29 COMB | AccuRAcY 304 DIVIDER AND 3| SAMPLE CONTROLLER 36 BUFFER COUNTER REFERENCE '6 CONTROLLER TIME COUNTER AUTOMATED MULTIPLE SAMPLE PROCESSING FOR WELL TYPE RADIOACTIVITY COUNTERS This invention relates to a system for vastly reducing the processing time required for comparative radioactivity analysis of samples in a set of samples.

The invention is particularly applicable to radioactivity measurement where a great multiplicity of samples are processed and compared against reference samples. Typically, such tests are based on a binding reaction between a ligand and a compatible binding agent in which one member is accompanied by tracer molecules labeled with a radioisotope, usually radioiodine 1-125. The test is used to detect substances which are in such low concentrations or small amounts that they can be quantified best through the use of radiotracers. Principal uses of such tests include quantification of hormones, enzymes, drugs, proteins, viruses, microbial toxins, and food additives. Knowledge of the quantity present or the absence of these substances in the tissues, fluids, and excretions of biological subjects is quite useful in diagnosing and treating certain illnesses. In particular, radioimmunoassay, a principal technique in such tests, is one of the most promising methods of detecting the presence of various cancers in their early stages in a living subject.

It is an object of this invention to realize a cost savings in the processing of a multiplicity of samples in radioactivity measurement. This cost savings is realized through a reduction in the amount of time which must be devoted to processing a large number of samples.

A related object is to increase the speed with which a given number of samples may be processed. This speed is realized from the concurrent counting of replicate samples and from the elimination of the need for mathematical calculations by the laboratory personnel.

A further object is to increase the reliability of the measurement of radioactivity in liquified samples by eliminating manual arithemtic calculations and the resultant errors that follow therefrom. Reliability may also be increased by designing the components of the radioactivity measuring device for use with a particular radioisotope.

A further object is to increase the simplicity of the performance of radioactivity analysis of a multiplicity of sample specimens. This may be achieved, particularly in the field of radioimmunoassay, by designing the radioactivity measuring device to analyze and extract information from reference samples, and to analyze subsequent samples and compare them with the reference samples first analyzed.

In a broad aspect this invention is a method of determining quantitatively the presence of a substance exist: ing at least partially in a radioactive form in each set of a series of sets of replicate samples utilizing a scintillation counter having a comparison means and an output means comprising concurrently analyzing each sample in a set of replicate samples in said scintillation counter to determine the radioactivity level in each sample, generating electrical signals representative of the radioactivity level in each sample concurrently measured, combining the electrical signals for the radioactivity levels of all the replicate samples in each set, comparing said combined electrical signals with an electrical signal representing the radioactivity level of a reference set of samples utilizing said comparison means, and indicating the results of the comparison on said output means.

In an alternative form of the invention, the samples in the sample set which are concurrently analyzed need not be replicate samples. In the cases where a single sample is deemed to be sufficiently reliable in level of radioactivity, the samples are grouped in a sample set merely to increase the speed of processing. In this case, the radioactivity level of each sample in the set is concurrently measured and a representative electrical signal is generated for each of these samples. Each of these electrical signals is compared separately with an electrical signal representing the radioactivity level of a reference sample. The results of each of these comparisons are separately indicated.

In another broad aspect this invention is, in a scintillation counter having a rate monitor for measuring the level of radioactivity, an output means, and a counting well, the improvement comprising a plurality of separate scintillation crystal assemblies, each constructed to accomodate concurrently a proximately positioned sample in a set of samples, a plurality of photodetectors, one each being in optical communication with only one of the plurality of scintillation cryatal assemblies, signal combining means connected to the rate monitor for combining the signals of the plurality of photodetectors, reference memory means for storing a signal indicative of level of radioactivity of a reference, and comparison means for receiving signals from the rate monitor and the reference memory means for determining the ratio between the level of radioactivity of samples in the counting well and the level of radioactivity of the reference, and for transmitting the ratio so derived to the output means.

In the preferred embodiment for processing replicate samples, the invention further includes a signal combining means connected to the count tabulating means or rate monitor. The signal combining means combines the signals of the plurality of photodetectors. The reference memory means stores a signal representing the elapsed counting time for the reference samples for a number of radioactive events. The count tabulating means is constructed to receive signals from the reference memory means for measuring the ratio between the radioactivity rate for the reference samples and the radioactivity rate of each set of replicate samples.

Preferably, a residual memory means for storing a signal representing residual radioactivity is also included in the device ln this arrangement a residual subtracting means connected between the photodetectors and the count tabulating means reduces the tabulated radioactive event count according to the level of residual radioactivity.

The invention may be further illustrated by reference to the accompanying drawings.

FIG. 1 is a block diagram of a well type scintillation counter constructed according to this invention.

FIG. 2 is a sectional view of the counting well of the scintillation counter of FIG. 1.

FIG. 3 is a typical standard curve for radioimmunoassay.

Referring now to the drawings, there is illustrated a scintillation counter having an output means in the form of a readout device 9, and a counting well defined by the enclosing walls 23. Within the scintillation counter are positioned aplurality of separate scintillation crystal assemblies 2 optically shielded from each other by the counting walls 23 and the partition means 28. A photodetector 3 is positioned directly under each scintillation crystal assembly 2. Windows 27 are interposed between each crystal 2 and photodetector 3, while the remaining surface area of each of the crystals 2 is surrounded by a metallic moisture shield 24. One of each of a plurality of replicate samples 1 is positioned proximate to a scintillation crystal assembly 2 respectively associated therewith. In response to radiation from radioisotopes within the samples 1, scintillations of light are generated in the scintillation crystal assemblies 2. These light scintillations cause the photodetectors 3 to generate electrical signals. The number of electrical signals generated by each photodetector is representative of the radioactivity level in the sample associated therewith. The electrical signals pass through amplifiers 4 and pulse height analyzers 5. The pulse height analyzers allow passage of electrical signals within the energy level limits dictated by the radioisotope of interest. Qualifying electrical pulses are passed to the pulse counters 8a, 8b, and 8c, where they are counted and registered in the readout device 9. A residual subtracting means 6 is interposed between the output of each of the pulse height analyzers 5 and a summing device 7 to reduce the rate of electrical pulse passage through the pulse height analyzers 5 by an amount equal to the rate of residual radioactivity. subtracting means 6 may be a typical conventional device such as an up-down counter which will pass pulses to signal combining means 7 only when positive counts exceed negative counts. The negative, residual counts are initially set into subtracting means 6 by residual memory means 34. The residual radioactivity is equal to the sum of the instrument background and residual or non-specific binding radioactivity, in the case of radioimmunoassay samples and standards. The count rate passed to a counter from each photodetector 3 then reflects the count rate passed by the associated pulse height analyzer 5 less the residual radioactivity rate. The pulse amplitude windows in the pulse height analyzers 5 will be set to cover identical ranges as long as the same radioisotope of interest is utilized in all of the sample vials l.

Initially, in its operation, the scintillation detector is run in the reference measuring mode under the influence of the reference controller 13. Operation in the reference mode is for the purpose of obtaining and storing reference figures from reference samples. Information subsequently obtained from unknown samples in the sample mode is then compared with the corresponding reference figures obtained from the reference samples in the analysis of unknown samples. The reference mode may be manually altered so that the reference figures may reflect the ratios of: number of bound sites to maximum binding; number of free sites to maxi mum free sites; number of free sites to total free sites; or number of bound sites; to total binding sites. Usually the ratio of bound sites to maximum binding is the most useful figure.

The reference controller 13 provides an actuating signal to the time counter which, when a reset signal is present at the AND gate 20, resets and starts the time counter 15. A clock 14 generates clock pulses which are recorded in the time counter 15. The time counter 15 thereby serves as a timing means for determining and generating an electrical signal representing elapsed counting time. Time counter 15 passes this signal to the buffer counter 16 when the master counter 22 is full. Master counter 22 serves as a count tabulating means, and when it is full, an appropriate signal is generated on electrical lead 29 which removes one of the inputs to AND gate 18, thereby preventing further storage of clock pulses in the time counter 15. The same signal actuates buffer counter 16 through AND gate 17 to accept and store an electrical signal from time counter 15 by way of line 35. The buffer counter 16 serves as a reference memory means for storing a signal representing elapsed counting time for a reference sample, or set of samples, for the predetermined number of events which were required to fill the master counter 22. The predetermined number of events required to fill the master counter 22 may be less than the number of electrical pulses passed from the signal combining means 7 if an accuracy divider 10 is interposed between the signal combining means 7 and the master counter 22. The accuracy divider passes one pulse to the master counter 22 for each time that a whole positive number of pulses are received from the signal combining means 7. The accuracy divider 10 thereby effectively divides the composite electrical signal from the signal combining means 7 by a whole positive integer. For example, if 10,000 pulses per minute are passed from each of the photodetectors 3 (after reduction by residual subtracting means 6) to the pulse combining means 7 when the samples 1 are each reference samples in a reference set and each contain a maximum concentration of the radioactive form of the substance under investigation, and if the master counter 22 has four significant counting spaces, then it is logical to divide the composite signals from signal combining means 7 by the number 30. Thus, 30,000 pulses from signal combining means 7 produce a count of 1,000 in master counter 22 in a one minute interval. When the counter 22 is full, the time elapsed, as recorded in time counter 15, is transferred for storage to buffer counter 16. The elapsed time is also registered in the readout device 9.

The denominator set dial 11 may be used to manually change the denominator setting in accuracy divider 10 if desired.

After the reference mode of operation, the sample controller 12 is not actuated until the samples 1 have been lowered into the counting well and are positioned for counting therein. Thereafter the scintillation counter ceases to operate in the reference mode, and instead is automatically switched to operate in the sample mode under the influence of sample controller 12. The sample controller 12 initiates a signal through the OR gate 19 which, when combined with the reset signal from circuit 30, resets and starts the time counter 15 again.

In the sample mode of operation, the clock 14 passes pulses to the time counter 15 which begins recording the time elapsed. In addition, a comparison command is sent to the buffer counter 16 from sample controller 12 on line 36 so that as soon as the contents of the time counter 15 equals the contents of the buffer counter 16, which were stored from counting the reference sample set, then a signal is generated on lead 31 from the buffer counter 16. This signal is passed as an input to AND gate 21 along with an input from the sample controller 12. AND gate 21, when actuated, generates a shut-off signal to the master counter 22 so that the count then present in the master counter 22 records the number of radioactive events.

Once the shut-off signal has been received by master counter 22 from AND gate 21 in the sample mode of operation, and the count from master counter 22 and the time from buffer counter 16 have been registered in readout device 9, readout device 9 generates a reset signal on circuit 30 which resets the time-counter as well as the counting means 8a, 8b, and 8c, and the master counter 22. Another start up signal is not generated by the sample controller 12 until the samples 1 have been removed from the scintillation counting well and subsequent samples placed in the counting well. At this point in time, as at the beginning of the reference mode, buffer counter 16 contains the time elapsed for counting during the reference mode and the master counter 22 and the time counter 15 are reset to zero. By being reset, master counter 22 may also be treated as containing the reference sample count, since reference sample measurement terminated at overflow at which time there was also a zero reading in master counter 22. The counting cycle is then repeated indefinitely in the sample mode until no more samples are available or until a signal switching to the reference mode of operation is received. Such a signal may be eigher automatic or manual. When a signal switching to the reference mode is received, the residual memory means 34 is cleared and is reset by the new references. The number in the master counter 22 is not only a raw count of pulses for the same length of time that elapsed during the counting of the reference sample set, but is also a ratio of the quantity of the radioisotope of interest in a subsequent sample set to the quantity of the same radioisotope present in the samples in the reference sample set. The electrical signal from the signal combining means 7 is thereby compared with an electrical signal representing the radioactivity level of a reference set of samples utilizing a master counter 22 which serves as a comparison means. The result of each comparison is in the form of a ratio which is indicated at the output means 9. This ratio may be used to determine the quantity of a substance of interest which exists either in a radioactive form, or, by applying well known principles of isotope dilution analysis, the quantity of both radioactive and non-radioactive forms. For example, with reference to FIG. 3, the ratio registered in readout device 9 represents the percentage fraction of tracer bound antibodies in a radioimmunoassay to determine the concentrations of ligand. Of course the curve of FIG. 3 is only an exemplary curve, as the standard curves in radioimmunoassay must be recalculated for each new batch of samples or for a new scintillation device if the same set of samples is to be used in more than one scintillation detector.

This invention may be explained further by describing the particular procedure involved in an exemplary use of the device to make specific quantitative determinations of a substance of interest.

As previously discussed, this invention has particular application to radioimmunoassay samples and standards. In radioimmunoassay, the reaction that takes place is usually between a ligand, typically an antigen, and an antibody or other binding agent, with the antigen typically being the substance of interest. The antigen molecules will combine with the antibodies until the antibody binding sites are saturated. The excess antigen present will remain free in solution in an uncombined state. The substance being investigated, the antigen, exists in both radioactive and non-radioactive forms in all of the reference samples and the unknown samples. Typically, the radioactive form of the antigen is created by reacting a stock preparation of antigen with iodine, preferably 1-1 25. Iodine reacts very readily with most antigens in a manner which does not later interfere with the antigen-antibody reaction. Therefore, at least some of the antigens are combined with radioactive iodine, to form a tracer labeled antigen. The antibodies react equally with the tracer labeled antigens and the cold, or non-radioactive antigens, which compete for antibody binding sites, so that the amount of radioactivity in the combined substances is indicative of the total quantity of antigen present. The number of radioactive tracer antigen molecules competing successfully, thus becoming bound, will, of course, depend upon the extent to which they have been diluted by non-radioactive antigen molecules. The higher the concentration of antigen in the test samples, the smaller the fraction of radioactive antigen tracer molecules bound by antibody. By constructing a curve based on tracer binding in a series of known, graded concentrations of unlabeled antigen, one then has the basis for detennining concentrations of antigens in unknown test samples.

A reference sample is prepared which is best described as a maximum binding reference, wherein a maximum of the radioactive form of the antigen is bound by reason of the presence of only a minimum of the non-radioactive form of the antigen. The maximum binding reference sample, or maximum binding reference sample set in the case of replicate samples, serves as the principal reference in radioimmunoassay.

In determining quantitatively the presence of antigen in replicate radioimmunoassay samples, the level of radioactivity in each sample in the sample set is concurrently measured in the scintillation counter. Separate electrical signals are generated representing the level of radioactivity in each replicate sample in the common set. A uniform adjustment of each of the electrical signals is made to nullify the influence of residual radiation on the electrical signals. The electrical signals are thereafter combined, usually by addition, to produce a composite electrical signal representing the level of radioactivity in the set of replicate samples. Alternatively, the signals representing each replicate sample may be combined by summation together to form an electrical signal reflecting the total radioactive events occurring in the entire set of replicate samples, followed by division of the total radioactive events by the number of replicate samples measured in the set to obtain an average level of radioactivity. In each case, a composite signal is produced and is compared against an electrical signal representing a composite radioactivity level of a reference standard sample set. The results of this comparison are indicated at some output ing the counting. Each set of subsequent unknown replicate samples may be analyzed by counting the same number of radioactive events while recording the time elapsed during the counting process. A ratio of radioactivity may be obtained by dividing the time elapsed during counting of the reference samples by the time elapsed during counting of each of the sets of unknown samples. This resultant ratio is a ratio of the quantity of the tracer labeled substance present in each set of unknown samples to the quantity of the tracer labeled substance present in the reference. Preferably, all of the measured times elapsed are increased by a uniform increment to negate the effects of radioactive events attributable to residual radiation.

Replicate samples are sometimes deemed unnecessary in a radioimmunoassay process where the extent of the reaction of the substances involved is particularly reliable and reproduceable for given quantities of the substances. In these cases, the advantages of this particular invention may be best obtained by processing different unknown samples in each sample set, rather than by processing replicate samples. In this form of practice of the invention, the scintillation counter must be provided with a plurality of comparison means and output means. The level of radioactivity of each of the diverse samples in the sample set is concurrently compared with the level of radioactivity in a maximum binding reference sample saturated to the maximum extent with the tracer labeled substance to obtain ratios for each of the unknown samples in the sample set. Each of these ratios is separately indicated in one of the plurality of output means.

The preferred embodiments and the forms of the practice of this invention heretofore described are for purposes of illustration only, and are not intended to limit the scope of the invention, as the invention is applicable to any radiation counting system wherein a multiplicity of samples are to be counted.

1 claim:

1. A method of automatically comparing the amount of a substance existing at least partially in a radioactive form in each set of a series of sets of replicate samples with the amount of said same substance present in reference samples using a scintillation counter comprising the following steps:

a. operating said scintillation counter in a reference mode of operation to analyze reference samples by generating electrical signals representing the occurrence of detected radioactive events in said reference samples,

b. counting said electrical signals,

c. determining the time rate of detection of radioactive events in said reference samples,

d. storing in said scintillation counter a signal representing the time rate of detection of radioactive events in said reference samples,

e. subsequently operating said scintillation counter in a sample mode to successively analyze each of the aforesaid sets of replicate samples to concurrently generate electrical signals representing the occurrence of detected radioactive events in each sample in a replicate sample set,

f. combining the electrical signals for the radioactive events detected in each sample within a replicate sample set, to produce signals representing the time rate of detection of radioactive events in each replicate sample set, and

g. generating output signals representing a comparison of the time rate of detection of radioactive events in each replicate sample set with the time rate of detection of radioactive events in said reference samples.

2. The method of claim 1 wherein the time rate of detection of radioactive events in said reference samples is determined by counting the number of electrical signals associated with radioactive events in said reference samples for a measuring interval of time, and wherein the time rate of detection of radioactive events in each replicate sample set is determined by counting the number of combined electrical signals associated with radioactive events in each of said replicate sample sets for the same measuring interval of time, and said output signals are representations of the ratios of the number of combined electrical signals associated with each replicate sample set to the counted number of electrical signals associated with said reference samples.

3. The method of claim 1 wherein the time rate of detection of radioactive events in said reference samples is determined by measuring the time elapsed during counting a predetermined number of electrical signals associated with radioactive events in said reference samples, and further comprising counting the same predetermined number of electrical signals associated with each of said replicate sample sets while measuring the time elapsed during counting of the samples in each replicate sample set, and wherein said output signals are representations of the ratios of time elapsed during counting of said reference samples to time elapsed during counting of each of said replicate sample sets, thereby indicating the ratios of the quantities of the aforesaid substance present in each of said replicate sample sets to the quantity of the aforesaid substance present in said reference samples.

4. The method of claim 3 wherein said electrical signals associated with samples within a replicate sample set are combined by summing said signals from each of the samples of a replicate sample set, and dividing the summed signals by the number of samples in the aforesaid replicate sample set to arrive at an average time rate of detection of radioactive events in the aforesaid replicate sample set.

5. The method of claim 3 further comprising increasing the measured times elapsed by a uniform increment to negate the effects of radioactive events attributable to residual radiation.

6. In a method of analyzing reference samples and subsequently analyzing each sample in a series of sets of replicate samples in a scintillation counter to obtain a comparison of the amount of a substance existing at least partially in a radioactive form in each of said sets of replicate samples with the amount of said same substance present in said reference samples, the improvement comprising, for each replicate sample set, concurrently analyzing all of the samples of a replicate sample set, combining electrical signals indicative of the results of analysis of all of the samples of a replicate sample set, and generating output signals for each replicate sample set which indicate separately the results of comparison of each replicate sample set with the reference samples.

7. The method of claim 6 wherein a uniform adjustment is made in the results of analysis of the reference samples and the results of analysis of each of the replicate sample sets to nullify the influence of residual radiation on said results.

8. In a method of analyzing a reference sample and subsequently analyzing each sample specimen in a series of sets of sample specimens in a scintillation counter to obtain a comparison of the amount of a substance existing at least partially in a radioactive form in each sample specimen in said sets of sample specimens with the amount of said same substance present in said reference sample, the improvement comprising, for each sample specimen set, concurrently analyzing all of the sample specimens of a sample specimen set, and generating separate output signals for each sample specimen in said sets of sample specimens to separately indicate the results of comparison of each sample specimen with the reference sample.

9. The method of claim 8 further comprising analyzing said reference sample by measuring the time elapsed during counting a predetermined number of electrical signals associated with radioactive events in said reference sample, analyzing each of said sample specimens in said set of sample specimens by counting the same predetermined number of electrical signals associated with radioactive events in said sample specimens in said sample specimen sets, and wherein said output signals are representations of the ratios of time elapsed during analysis of said reference sample to time elapsed during analysis of each of said sample specimens in each of said sets of sample specimens, thereby indicating the ratios of the quantities of the aforesaid substance present in each of said sample specimens to the quantity of the aforesaid substance present in said reference sample.

10. In a scintillation counter having a counting well for accommodating samples positioned therein, the improvement comprising a plurality of separate scintillation crystal assemblies in said counting well, each constructed to concurrently accommodate a single one of a set of samples in proximate location thereto, a plurality of photodetectors for generating electrical signals each being in optical communication with a different one of said scintillation crystal assemblies, and each scintillation crystal assembly being in optical communication with only one photodetector, count tabulating means for receiving and counting signals from said photodetectors, timing means for recording elapsed sample counting time, said count tabulating means and said timing means being operative together to produce a signal indicative of time rate of detection of radioactive events from said scintillation crystal assemblies, reference memory means for storing a signal indicative of time rate of detection of radioactive events in reference samples, and output means for receiving signals from said count tabulating means and said reference memory means to represent comparisons of the time rate of detection of radioactive events occurring in subsequent samples with the time rate of detection of radioactive events occurring in reference samples.

11. The scintillation counter of claim 10 further comprising a signal combining means connected to all of said photodetectors and to said count tabulating means.

12. The scintillation counter of claim 10 further comprising a residual memory means for storing a signal representing residual radioactivity, and a residual subtracting means connected between said photodetectors and said count tabulating means for reducing the tabulated radioactive event count according to the level of residual radioactivity.

13. In a scintillation counter for analyzing specimen samples having a means for determining time rate of detection of radioactive events, an output means, and a counting well, the improvement comprising a plurality of separate scintillation crystal assemblies in said counting well, each constructed to accommodate concurrently a proximately positioned sample in a set of samples, a plurality of photodetectors for providing signals to said means for determining time rate of detection of radioactive events, each photodetector being in optical communication with only one of said plurality of scintillation crystal assemblies, and each scintillation crystal assembly being in optical communication with only one photodetector, reference memory means, a reference controller for actuating said reference memory means to store a signal from said means for determining time rate of detection of radioactive events in reference samples, and a sample controller for actuating said reference memory means and said means for determining time rate of detection of radioactive events to provide signals to said output means to indicate the ratio of the time rate of detection of radioactive events in specimen samples to the time rate of detection of radioactive events in reference samples. 

1. A method of automatically comparing the amount of a substance existing at least partially in a radioacTive form in each set of a series of sets of replicate samples with the amount of said same substance present in reference samples using a scintillation counter comprising the following steps: a. operating said scintillation counter in a reference mode of operation to analyze reference samples by generating electrical signals representing the occurrence of detected radioactive events in said reference samples, b. counting said electrical signals, c. determining the time rate of detection of radioactive events in said reference samples, d. storing in said scintillation counter a signal representing the time rate of detection of radioactive events in said reference samples, e. subsequently operating said scintillation counter in a sample mode to successively analyze each of the aforesaid sets of replicate samples to concurrently generate electrical signals representing the occurrence of detected radioactive events in each sample in a replicate sample set, f. combining the electrical signals for the radioactive events detected in each sample within a replicate sample set, to produce signals representing the time rate of detection of radioactive events in each replicate sample set, and g. generating output signals representing a comparison of the time rate of detection of radioactive events in each replicate sample set with the time rate of detection of radioactive events in said reference samples.
 2. The method of claim 1 wherein the time rate of detection of radioactive events in said reference samples is determined by counting the number of electrical signals associated with radioactive events in said reference samples for a measuring interval of time, and wherein the time rate of detection of radioactive events in each replicate sample set is determined by counting the number of combined electrical signals associated with radioactive events in each of said replicate sample sets for the same measuring interval of time, and said output signals are representations of the ratios of the number of combined electrical signals associated with each replicate sample set to the counted number of electrical signals associated with said reference samples.
 3. The method of claim 1 wherein the time rate of detection of radioactive events in said reference samples is determined by measuring the time elapsed during counting a predetermined number of electrical signals associated with radioactive events in said reference samples, and further comprising counting the same predetermined number of electrical signals associated with each of said replicate sample sets while measuring the time elapsed during counting of the samples in each replicate sample set, and wherein said output signals are representations of the ratios of time elapsed during counting of said reference samples to time elapsed during counting of each of said replicate sample sets, thereby indicating the ratios of the quantities of the aforesaid substance present in each of said replicate sample sets to the quantity of the aforesaid substance present in said reference samples.
 4. The method of claim 3 wherein said electrical signals associated with samples within a replicate sample set are combined by summing said signals from each of the samples of a replicate sample set, and dividing the summed signals by the number of samples in the aforesaid replicate sample set to arrive at an average time rate of detection of radioactive events in the aforesaid replicate sample set.
 5. The method of claim 3 further comprising increasing the measured times elapsed by a uniform increment to negate the effects of radioactive events attributable to residual radiation.
 6. In a method of analyzing reference samples and subsequently analyzing each sample in a series of sets of replicate samples in a scintillation counter to obtain a comparison of the amount of a substance existing at least partially in a radioactive form in each of said sets of replicate samples with the amount of said same substaNce present in said reference samples, the improvement comprising, for each replicate sample set, concurrently analyzing all of the samples of a replicate sample set, combining electrical signals indicative of the results of analysis of all of the samples of a replicate sample set, and generating output signals for each replicate sample set which indicate separately the results of comparison of each replicate sample set with the reference samples.
 7. The method of claim 6 wherein a uniform adjustment is made in the results of analysis of the reference samples and the results of analysis of each of the replicate sample sets to nullify the influence of residual radiation on said results.
 8. In a method of analyzing a reference sample and subsequently analyzing each sample specimen in a series of sets of sample specimens in a scintillation counter to obtain a comparison of the amount of a substance existing at least partially in a radioactive form in each sample specimen in said sets of sample specimens with the amount of said same substance present in said reference sample, the improvement comprising, for each sample specimen set, concurrently analyzing all of the sample specimens of a sample specimen set, and generating separate output signals for each sample specimen in said sets of sample specimens to separately indicate the results of comparison of each sample specimen with the reference sample.
 9. The method of claim 8 further comprising analyzing said reference sample by measuring the time elapsed during counting a predetermined number of electrical signals associated with radioactive events in said reference sample, analyzing each of said sample specimens in said set of sample specimens by counting the same predetermined number of electrical signals associated with radioactive events in said sample specimens in said sample specimen sets, and wherein said output signals are representations of the ratios of time elapsed during analysis of said reference sample to time elapsed during analysis of each of said sample specimens in each of said sets of sample specimens, thereby indicating the ratios of the quantities of the aforesaid substance present in each of said sample specimens to the quantity of the aforesaid substance present in said reference sample.
 10. In a scintillation counter having a counting well for accommodating samples positioned therein, the improvement comprising a plurality of separate scintillation crystal assemblies in said counting well, each constructed to concurrently accommodate a single one of a set of samples in proximate location thereto, a plurality of photodetectors for generating electrical signals each being in optical communication with a different one of said scintillation crystal assemblies, and each scintillation crystal assembly being in optical communication with only one photodetector, count tabulating means for receiving and counting signals from said photodetectors, timing means for recording elapsed sample counting time, said count tabulating means and said timing means being operative together to produce a signal indicative of time rate of detection of radioactive events from said scintillation crystal assemblies, reference memory means for storing a signal indicative of time rate of detection of radioactive events in reference samples, and output means for receiving signals from said count tabulating means and said reference memory means to represent comparisons of the time rate of detection of radioactive events occurring in subsequent samples with the time rate of detection of radioactive events occurring in reference samples.
 11. The scintillation counter of claim 10 further comprising a signal combining means connected to all of said photodetectors and to said count tabulating means.
 12. The scintillation counter of claim 10 further comprising a residual memory means for storing a signal representing residual radioactivity, and a residual subtracting means connected between said photodetectors and said count Tabulating means for reducing the tabulated radioactive event count according to the level of residual radioactivity.
 13. In a scintillation counter for analyzing specimen samples having a means for determining time rate of detection of radioactive events, an output means, and a counting well, the improvement comprising a plurality of separate scintillation crystal assemblies in said counting well, each constructed to accommodate concurrently a proximately positioned sample in a set of samples, a plurality of photodetectors for providing signals to said means for determining time rate of detection of radioactive events, each photodetector being in optical communication with only one of said plurality of scintillation crystal assemblies, and each scintillation crystal assembly being in optical communication with only one photodetector, reference memory means, a reference controller for actuating said reference memory means to store a signal from said means for determining time rate of detection of radioactive events in reference samples, and a sample controller for actuating said reference memory means and said means for determining time rate of detection of radioactive events to provide signals to said output means to indicate the ratio of the time rate of detection of radioactive events in specimen samples to the time rate of detection of radioactive events in reference samples. 